Bleed valves of various types have been placed in reservoirs and fluid return lines of hydraulic systems. These values serve to differentiate between fluid in gaseous form and fluid in liquid form and vent, or bleed, either the gaseous or liquid form. Many of these valves have been large and often have been manually operated. Compact, automatic bleed valves for such systems have been described in U.S. Pat. Nos. 4,524,793 and 4,813,446 to Silverwater, and copending U.S. application Ser. No. 07/887,836, all assigned to the assignee of the present application.
A general theory of operation of these automatic bleed valves is explained in the '793 patent. A capillary and orifice placed in series in a fluid channel to cause the pressure distribution along the channel between a high pressure point at the reservoir end of the valve and a low pressure point at the discharge end of the valve to vary depending upon the phase of the fluids flowing in the channel. This theory is based upon the known fact that, in such an arrangement, a steeper pressure gradient will occur over the orifice in the case of gaseous phase flow and, conversely, a steeper gradient will be observed over the capillary portion of such a channel during liquid phase flow. The variation in the pressure distribution in the channel may be utilized to control the opening and closing of a differentiating valve, depending upon the phase of flow through the valve. The preferred embodiment disclosed in the '793 patent is automatic and, thus, mitigates the need for constant operator vigilance.
Thus, each of the aforementioned bleed valves utilizes a series connection of a capillary and an orifice and depends on the fact that a different pressure drop across the capillary occurs when the fluid is flowing in a gas as opposed to a liquid phase. The pressure drop across the capillary is related to the viscosity of the fluid flowing through the capillary and both the spring tension and the capillary size must be adjusted to the particular fluid viscosity.
Many liquids used in hydraulic applications (such as oil) have a viscosity that varies greatly with temperature. Because the design of these conventional bleed valves must be directed to a particular viscosity, they may not work as reliably in environments in which wide temperature variations result in changes in the viscosity of the fluid flowing through the bleed valve. Consequently, this problem is particularly acute when the device is utilized in an environment with very wide temperature swings such as in aerospace applications.
Conventional bleed valves are also difficult to miniaturize and manufacture because of the length and diameter of the capillary.